The present invention relates to multifocal ophthalmic lenses. Such lenses are well known; they provide an optical power which varies continuously as a function of the position on the lens; typically when a multifocal lens is mounted in a frame, the power in the bottom of the lens is greater than the power in the top of the lens.
In practice, multifocal lenses often comprise an aspherical face, and a face which is spherical or toric, machined to match the lens to the wearer""s prescription. It is therefore usual to characterize a multifocal lens by the surface parameters of its aspherical surface, namely at every point a mean sphere S and a cylinder.
The mean sphere S is defined by the following formula:   S  =                    n        -        1            2        ⁢          (                        1                      R            1                          +                  1                      R            2                              )      
where R1 and R2 are the minimum and maximum radii of curvature, expressed in meters, and n is the refractive index of the lens material.
The cylinder is given, using the same conventions, by the formula:   C  =            (              n        -        1            )        ⁢          "LeftBracketingBar"                        1                      R            1                          -                  1                      R            2                              "RightBracketingBar"      
Such multifocal lenses adapted for vision at all distances are called progressive lenses. Progressive ophthalmic lenses usually comprise a far vision region, a near vision region, an intermediate vision region and a main meridian of progression passing through these three regions. French patent 2,699,294, to which reference may be made for further details, describes in its preamble the various elements of a progressive multifocal ophthalmic lens, together with work carried out by the assignee in order to improve the comfort for wearers of such lenses. In short, the upper part of the lens, which is used by the wearer for distance vision, is called the far vision region. The lower part of the lens is called the near vision region, and is used by the wearer for close work, for example for reading. The region lying between these two regions is called the intermediate vision region.
The difference in mean sphere between a control point of the near vision region and a control point of the far vision region is thus called the power addition or addition. These two control points are usually chosen on the main meridian of progression defined below.
For all multifocal lenses, the power in the various far, intermediate and near vision regions, independently of their position on the lens, is determined by the prescription. The latter may comprise just a power value for near vision or a power value for far vision and an addition, and possibly an astigmatism value with its axis and prism.
For progressive lenses, a line called the main meridian of progression is a line used as an optimization parameter; this line is representative of the strategy for using the lens by the average wearer. The meridian is frequently a vertical umbilical line on the multifocal lens surface, i.e. alignment for which all points have zero cylinder. Various definitions have been proposed for the main meridian of progression.
In a first definition, the main meridian of progression is constituted by the intersection of the aspherical surface of the lens and an average wearer""s glance when looking straight ahead at objects located in a meridian plane, at different distances; in this case, the meridian is obtained from a definitions of the average wearer""s posturexe2x80x94point of rotation of the eye, position of the frame, angle the frame makes with the vertical, near vision distance, etc.; these various parameters allow the meridian to be drawn on the surface of the lens. French patent application 2,753,805 is an example of a method of this type in which a meridian is obtained by ray tracing, taking account of the closeness of the reading plane as well as prismatic effects.
A second definition consists in defining the meridian using surface characteristics, and notably isocylinder lines; in this context, an isocylinder line for a given cylinder value represents all those points that have a given cylinder value. On the lens, horizontal segments linking 0.50 diopter isocylinder lines are traced, and the mid-points of these segments are considered. The meridian is close to these mid-points. We can thus consider a meridian formed from three straight line segments which are the best fit to pass through the middles of the horizontal segments joining the two isocylinder lines. This second definition has the advantage of allowing the meridian to be found from measurement of lens surface characteristics, without advance knowledge of the optimization strategy that will be used. With this definition, isocylinder lines for half the power addition can be considered instead of considering 0.50 diopter isocylinder lines.
A third definition of the meridian is proposed in the assignee""s Patents. To best satisfy the requirements of presbyopic spectacle wearers and improve progressive multifocal lens comfort, the assignee has proposed adapting the form of the main meridian of progression as a function of power addition, see French patent applications 2,683,642 and 2,683,643. The meridian in those patent applications is formed by three segments forming a broken line. Starting from the top of a lens, the first segment is vertical and has as its lower end, the mounting center (defined below). The top point of the second segment is located at the mounting center and makes an angle xcex1 with the vertical which is a function of power addition, for example
xcex1=f1(A)=1.574.A2xe2x88x923.097.A+12.293.
The second segment has a lower end at a vertical distance on the lens which is also dependent on power addition; this height h is for example given by
h=f2(A)=0.340.A2xe2x88x920.425.Axe2x88x926.422;
this formula gives a height in mm, in a reference frame centered on the lens center. The upper end of the third segment corresponds to the point at which the lower end of the second segment is located, and it makes an angle xcfx89 with the vertical which is a function of power addition, for example
xcfx89=f3(A)=0.266.A2xe2x88x920.473.A+2.967.
In this formula, as in the preceding ones, the numerical coefficients have dimensions suitable for expressing the angles in degrees and the height in mm, for a power addition in diopters. Other relations apart from this can obviously be used for defining a 3-segment meridian.
A point, called the mounting center, is commonly marked on ophthalmic lenses, whether they are progressive or not, and is used by the optician for mounting lenses in a frame. From the anthropometric characteristics of the wearerxe2x80x94pupil separation and height with respect to the framexe2x80x94the optician machines the lens by trimming the edges, using the mounting center as a control point. In lenses marketed by the assignee, the mounting center is located 4 mm above the geometric center of the lens; the center is generally located in the middle of the micro-etchings. For a lens correctly positioned in a frame, it corresponds to a horizontal direction of viewing, for a wearer holding his/her head upright.
French patent application serial number 0,006,214 filed May 16, 2000 tackles the problem of mounting progressive multifocal lenses in frames of small size: it can happen, when such lenses are mounted in small frames, that the lower portion of the near vision region is removed when the lens is machined. The wearer then has correct vision in the far and intermediate vision regions, but suffers from the small size of the near vision region. The wearer will have a tendency to use the lower part of the intermediate vision region for close work. This problem is particularly acute in view of the current fashion trend towards frames of small size.
Another problem encountered by wearers of progressive multifocal lenses is that of fatigue when performing prolonged work in close or intermediate vision. The near vision region of a progressive lens is indeed located in the bottom part of the lens, and prolonged use of the near vision region can produce fatigue in some spectacle wearers.
One last problem is that of wearer adaptation to such lenses. It is known that spectacle wearers and notably young presbyopic people usually require a period of adaptation to progressive lenses before being able to appropriately use the various regions of the lens for corresponding activities. This problem of adaptation is also encountered by people who formerly wore bifocal lenses; such lenses have a special near vision portion the upper part of which is generally located 5 mm below the geometric center of the lens. Now, in conventional progressive lenses, the near vision region is generally situated lower; even if it is difficult to exactly pinpoint the limit between the intermediate vision region and the near vision region, a wearer would suffer significant fatigue by using progressive lenses for near vision at 5 mm below the mounting center.
The invention proposes a solution to these problems by providing a lens of generalized optical design, suited to all situations. It provides in particular a lens able to be mounted in small size frames, without the near vision region getting reduced. It also improves wearer comfort with prolonged use of the near vision or intermediate vision regions. It makes it easier for younger presbyopic wearers and former wearers of bifocal lenses to adapt to progressive lenses. More generally, the invention is applicable to any lens having a rapid variation in power.
More precisely, the invention provides a progressive multifocal ophthalmic lens comprising an aspherical surface with at every point thereon a mean sphere and a cylinder, a far vision region, an intermediate vision region and a near vision region, a main meridian of progression passing through said three regions, a power addition equal to a difference in mean sphere between a near vision region control point and a far vision region control point, a progression length less than 12 mm, progression length being equal to the vertical distance between a mounting center and a point on the meridian where mean sphere is 85% of the power addition value greater than mean sphere at the far vision control point, in which the ratio between
firstly, the product of cylinder times the norm of sphere gradient, and
secondly, the square of power addition is less than 0.08 mmxe2x88x921 at every point within a 40 mm diameter disc centered on the center of the lens, and in which cylinder within that part of the disc situated above the mounting center is less than 0.5 times power addition.
In one embodiment, a ratio between
firstly, the integral of the product of cylinder times the norm of sphere gradient, on a 40 mm diameter circle centered on the center of the lens, and
secondly, the product of the area of the circle, power addition and a maximum value of the norm of sphere gradient over that part of the meridian comprised within the circle, is less than 0.14.
In another embodiment, a ratio between
firstly, the integral of the product of cylinder times the norm of sphere gradient, on a 40 mm diameter circle centered on the center of the lens, and
secondly, the product of the area of this circle, power addition and a maximum value of the norm of sphere gradient on that part of the meridian comprised within the circle, is less than 0.16 times the ratio between
a maximum value of the norm of sphere gradient on that part of the meridian comprised within the circle; and
a maximum value for the norm of sphere gradient within the circle.
Preferably, the main meridian of progression is an umbilical line. It can also be substantially formed by the mid-points of horizontal segments joining lines formed by 0.5 diopter cylinder points, or be formed by three segments constituting a broken line.
In this latter case, the first segment is advantageously vertical and has the mounting center as its lower end. The upper end of the second segment can be formed by the mounting center, and the segment can make an angle xcex1, which is a function of power addition, with the vertical. In this case, angle xcex1 is given by
xcex1=f1(A)=1.574.A2xe2x88x923.097.A+12.293,
where A is power addition.
The second segment can have a lower end at a height h which is a function of power addition. In this case, the height h of the lower end of the second segment is preferably given, in mm, in a reference frame centered on the center of the lens by the function
h=f2(A)=0.340.A2xe2x88x920.425.Axe2x88x926.422,
where A is power addition.
The third segment can make an angle xcfx89 which is a function of power addition, with the vertical. The angle xcfx89 is preferably given by
xcfx89=f3(A)=0.266.A2xe2x88x920.473.A+2.967,
where A is power addition
Further characteristics and advantages of the invention will become more clear from the detailed description which follows of some embodiments of the invention provided by way solely of example, and with reference to the drawings.